Method for preparing single walled carbon nanotubes

ABSTRACT

Methods of preparing single walled carbon nanotubes are provided. Carbon containing gas is contacted with a supported metal catalyst under reaction conditions to yield at least 90% single walled carbon nanotubes and at least 1 gram single walled carbon nanotubes/gram metal catalyst. The support material may be calcined at temperatures between 150 and 600° C., and may have at least one oxidized planar surface. Reaction conditions include less than 10 atmospheres pressure and less than 800° C.

CROSS REFERENCE INFORMATION

This is a continuation of U.S. Ser. No. 11/281,571 filed Nov. 16, 2005,currently pending which claims benefit to and priority of U.S.Provisional Application No. 60/630,946, filed Nov. 24, 2004, U.S.Provisional Application No. 60/630,781, filed Nov. 24, 2004 and U.S.Provisional Application No. 60/628,498, filed Nov. 16, 2004, each ofwhich is are hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to methods for preparing single walled carbonnanotubes. More specifically, the invention relates to methods forpreparing a bundle or a densely packed array of single walled carbonnanotubes under commercially viable reaction conditions.

2. Description of the Related Art

Carbon Nanotubes

This invention lies in the field of carbon nanotubes (also known asfibrils). Carbon nanotubes are vermicular carbon deposits havingdiameters less than 1.0μ, preferably less than 0.5μ, and even morepreferably less than 0.2μ. Carbon nanotubes can be either multi walled(i.e., have more than one graphene layer more or less parallel to thenanotube axis) or single walled (i.e., have only a single graphene layerparallel to the nanotube axis). Other types of carbon nanotubes are alsoknown, such as fishbone fibrils (e.g., wherein the graphene layers arearranged in a herringbone pattern, compared to the tube axis), etc. Asproduced, carbon nanotubes may be in the form of discrete nanotubes,aggregates of nanotubes (i.e., dense, microscopic particulate structurecomprising entangled carbon nanotubes) or a mixture of both.

Carbon nanotubes are distinguishable from commercially availablecontinuous carbon fibers. For instance, diameter of continuous carbonfibers, which is always greater than 1.0μ and typically 5 to 7μ, is farlarger than that of carbon nanotubes, which is usually less than 1.0μ.Carbon nanotubes also have vastly superior strength and conductivitythan carbon fibers.

Carbon nanotubes also differ physically and chemically from other formsof carbon such as standard graphite and carbon black. Standard graphite,because of its structure, can undergo oxidation to almost completesaturation. Moreover, carbon black is an amorphous carbon generally inthe form of spheroidal particles having a graphene structure, such ascarbon layers around a disordered nucleus. On the other hand, carbonnanotubes have one or more layers of ordered graphitic carbon atomsdisposed substantially concentrically about the cylindrical axis of thenanotube. These differences, among others, make graphite and carbonblack poor predictors of carbon nanotube chemistry.

It has been further accepted that multi walled and single walled carbonnanotubes are also different from each other. For example, multi walledcarbon nanotubes have multiple layers of graphite along the nanotubeaxis while single walled carbon nanotubes only have a single graphiticlayer on the nanotube axis.

The methods of producing multi walled carbon nanotubes also differ fromthe methods used to produce single walled carbon nanotubes.Specifically, different combinations of catalysts, catalyst supports,raw materials and reaction conditions are required to yield multi walledversus single walled carbon nanotubes. Certain combinations will alsoyield a mixture of multi walled and single walled carbon nanotubes.

As such, two characteristics are often examined in order to determinewhether such process will be commercially feasible for the production ofa desired carbon nanotube on an industrial scale. The first is catalystselectivity (e.g., will the catalyst yield primarily single wall carbonnanotubes or primarily multi-walled carbon nanotubes or other forms ofcarbon products?). The second is catalyst yield (e.g., weight of carbonproduct generated per weight of catalyst used).

Processes for forming multi walled carbon nanotubes are well known.E.g., Baker and Harris, Chemistry and Physics of Carbon, Walker andThrower ed., Vol. 14, 1978, p. 83; Rodriguez, N., J. Mater. Research,Vol. 8, p. 3233 (1993); Oberlin, A. and Endo, M., J. of Crystal Growth,Vol. 32 (1976), pp. 335-349; U.S. Pat. No. 4,663,230 to Tennent; U.S.Pat. No. 5,171,560 to Tennent; Iijima, Nature 354, 56, 1991; Weaver,Science 265, 1994; de Heer, Walt A., “Nanotubes and the Pursuit ofApplications,” MRS Bulletin, April, 2004; etc. All of these referencesare herein incorporated by reference.

Commercially known processes for forming multi walled carbon nanotubesare high in selectively (e.g., produces greater than 90% multi walledcarbon nanotubes in product) as well as yield (e.g., produces 30 poundsof multi walled carbon nanotube produce per pound catalyst).

Processes for making single walled carbon nanotubes are also known.E.g., “Single-shell carbon nanotubes of 1-nm diameter”, S Iijima and TIchihashi Nature, vol. 363, p. 603 (1993); “Cobalt-catalysed growth ofcarbon nanotubes with single-atomic-layer walls,” D S Bethune, C HKiang, M S DeVries, G Gorman, R Savoy and R Beyers Nature, vol. 363, p.605 (1993); U.S. Pat. No. 5,424,054 to Bethune et al.; Guo, T.,Nikoleev, P., Thess, A., Colbert, D. T., and Smally, R. E., Chem. Phys.Lett. 243: 1-12 (1995); Thess, A., Lee, R., Nikolaev, P., Dai, H.,Petit, P., Robert, J., Xu, C., Lee, Y. H., Kim, S. G., Rinzler, A. G.,Colbert, D. T., Scuseria, G. E., Tonarek, D., Fischer, J. E., andSmalley, R. E., Science, 273: 483-487 (1996); Dai., H., Rinzier, A. G.,Nikolaev, P., Thess, A., Colbert, D. T., and Smalley, R. E., Chem. Phys.Lett. 260: 471-475 (1996); U.S. Pat. No. 6,761,870 (also WO 00/26138) toSmalley, et. al; “Controlled production of single-wall carbon nanotubesby catalytic decomposition of CO on bimetallic Co—Mo catalysts,”Chemical Physics Letters, 317 (2000) 497-503; U.S. Pat. No. 6,333,016 toResasco, et. al., etc. All of these references are hereby by reference.

However, unlike multi walled carbon nanotube technology, currently knownprocesses for forming single walled carbon typically are unable to reachindustrially acceptable levels of selectivity and yield undercommercially viable reaction conditions. For example, in Maruyama, et.al. “Low-temperature synthesis of high-purity single walled carbonnanotubes from alcohol,” Chemical Physics Letters, 360, pp. 229-234(Jul. 10, 2002), herein incorporated by reference, a method is disclosedfor obtaining high purity single walled carbon nanotubes under vacuum orextremely low pressure (e.g., 5 Torr). Maintaining such extremely lowpressure conditions on an industrial scale reactor would not becommercially viable. Other references such as U.S. Pat. No. 6,333,016 toResasco also disclose high selectivity for single walled carbonnanotubes, but fail to show a commercially viable yield.

As such, there is a need for a method for producing single walled carbonnanotubes with industrially acceptable levels of activity, selectivityand yield under commercially viable reaction conditions.

SUMMARY OF THE INVENTION

The present invention provides methods of preparing single walled carbonnanotubes comprising contacting a carbon containing gas with a supportedmetal catalyst under reaction conditions at a selectivity of at least90% single walled carbon nanotubes and with a yield of at least 1 gramsingle walled carbon nanotubes/gram metal catalyst.

More specifically, the present invention provides a method for preparingsingle walled carbon nanotubes comprising the steps of calcining asupport material at temperatures between 150° C. to 600° C., saidsupport material having at least one planar surface; preparing asupported catalyst comprising a metal catalyst precursor and saidcalcined support material; optionally calcining and/or prereducing saidsupported catalyst; and contacting the supported catalyst with a carboncontaining gas at reaction conditions sufficient to produce at least 90%single walled carbon nanotubes in an amount greater than 1 gram singlewalled carbon nanotubes per gram metal catalyst; wherein the pressure insaid reaction conditions is greater than about one and less than about10 atmospheres and the temperature in said reaction conditions is lessthan 800° C. It is preferred that the planar surface of the supportmaterial be in an oxidized state. To oxidize the planar surface of thesupport material which does not have any oxide or oxygen groups, (i.e.,so as to have oxides present on the surface of the support material), itis preferred that the support material be oxidized prior to performingthe process of the present invention. Suitable oxidation temperaturesmay be greater than 1000° C.

In an alternative embodiment, the calcining step may be performed afterthe step of preparing a supported catalyst. In yet a further embodiment,the calcining step may be performed both before and after the step ofpreparing a supported catalyst.

Preferred metal catalysts include Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt,Cr, W, Mo, Mn, Ni or mixtures thereof. Preferred support materials arein the form of platelets, wafers or planar substrates and are made fromalumina (Al₂O₃), magnesia (MgO), silica (SiO₂), Mg(Al)O_(x), ZrO₂,molecular sieve zeolite, glass, quartz, clay, hydrotalcite, talc,aluminum foil or silicon.

Preferred reaction temperature range is 400 to 800° C., more preferred500-750° C., even more preferred 550 to 650° C. Preferred reactionpressure range is 0.5 to 10 atm, more preferred 1 to 5 atm, even morepreferred 1 to 2 atm.

It should be understood that reagent gases are necessarily supplied at apressure slightly in excess of the reaction zone pressure in order thatthey flow, without the aid of compression or other motive force into thereactor.

The support material may optionally be subjected to plasma treatmentbefore being used to prepare the supported catalyst. Plasmas which maybe used include those based on F₂, O₂, NH₃, He, N₂ and H₂, otherchemically active or inert gases, other combinations of one or morereactive and one or more inert gases or gases capable of plasma-inducedpolymerization such as methane, ethane or acetylene.

In those systems where the source of carbon contains oxygen, preferredconditions also include maintaining a favorable oxidation potential inthe reaction zone during the growth of the single walled carbonnanotubes by controlling the partial pressure of an oxidizing gas suchas molecular oxygen, carbon dioxide or water. Where the source of carbonis a hydrocarbon, it is advantageous to maintain a level of hydrogen inthe reaction gas in excess of the stoichiometric amount in the reactiontaking place.

The invention also includes methods, systems and catalyst configurationswhich facilitate the harvesting of single walled carbon nanotubes fromthe catalyst either in the reaction zone or in a subsequent separationzone. Preferred catalyst particle configurations are described.

Other improvements which the present invention provides over the priorart will be identified as a result of the following description whichsets forth the preferred embodiments of the present invention. Thedescription is not in any way intended to limit the scope of the presentinvention, but rather only to provide a working example of the presentpreferred embodiments. The scope of the present invention will bepointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of the process for making a dense array ofsingle walled carbon nanotubes on a planar substrate.

FIG. 2 displays representative Raman spectra of products formed at 600°C. and 700° C. in accordance with Examples 4-6.

FIG. 3 displays scanning electron micrographs (SEM) of the dense arrayof single walled carbon nanotubes prepared at 600° C. in accordance withthe present invention.

FIG. 4 displays transmission electron micrographs (TEM) of the densearray of single walled carbon nanotubes prepared at 600° C. inaccordance with the present invention.

FIG. 5 displays a Raman spectrum of the products obtained in accordancewith Example 9.

FIG. 6A-C displays the Raman spectra of the products obtained inaccordance with Example 10.

FIG. 7A-E displays the Raman spectra of the products obtained inaccordance with Example 11.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a new process for producing single walledcarbon nanotubes which results in industrially acceptable levels ofselectivity and yield under commercially viable reaction conditions.

In the preferred embodiment, there is provided a method for preparingsingle walled carbon nanotubes comprising contacting a carbon containinggas with a supported metal catalyst under reaction conditions to yieldat least 90% single walled carbon nanotubes and at least 1 gram singlewalled carbon nanotubes/gram metal catalyst. Preferred reactionconditions include less than 800° C. and less than 10 atmospherespressure.

The reaction can be conducted in any conventional reactor used toprepare carbon nanotubes.

The single walled carbon nanotubes produced in accordance with thepreferred embodiment are typically free of any pyrolytically depositedamorphous carbon. The single walled carbon nanotubes have diametersranging from 0.5 nanometer to 10 nanometers, preferably less than 5nanometers, more preferably between 0.5 and 1 nanometer.

Furthermore, single walled carbon nanotubes may be grown as individualnanotubes or as aggregates of nanotubes (i.e., dense, microscopicparticulate structure comprising entangled carbon nanotubes) or amixture of both. Due to the high nucleation efficiency, the methods ofthe preferred embodiment permit single walled carbon nanotubes to begrown as densely packed arrays, bundles or ropes of single walled carbonnanotubes, or so-called “single walled nanotube forest.” A “singlewalled nanotube forest” may comprise uniform or non-uniformsubstructures. For example, a forest may comprise multiple ropes ofaligned single walled carbon nanotubes, and said ropes may havediameters of 2-20 nm, more preferably less than 10 nm. In the preferredembodiment, all of the individual single walled carbon nanotubesproduced have similar or substantially identical diameters, and all thesubstructure ropes have similar or substantially identical diameters aswell. The density of the array, bundle, rope or forest may be between10¹⁶ to 10¹⁸ nanotubes/m². In one embodiment, the arrays or forests ofsingle walled carbon nanotubes may be aligned parallel or substantiallyparallel to each other.

In one embodiment, the present process results in the growth of anarray, bundle, rope or forest of single walled carbon nanotubes whereinat least 50% of the exposed surface area of the metal catalyst arecovered with bases of single walled nanotubes. In another embodiment,the present process yields a nucleation efficiency greater than 75%.

Carbon Containing Gas

The carbon containing gas can be any gaseous carbon source such as a C₁through C₆ compound having as hetero atoms H, O, N, S or Cl, optionallymixed with hydrogen. Carbon monoxide is preferred. Other useful carboncontaining gases include, but are not limited to, unsaturated andsaturated aliphatic hydrocarbons such as methane, ethane, propane,butane, hexane, ethylene, acetylene, propylene; oxygenated organiccompounds such as acetone; aromatic hydrocarbons such as toluene,benzene and napthalene. Alcohols such as methanol, ethanol, propanol,etc. may also be used.

The carbon containing gas can be mixtures of any of the previouslymentioned gases or can further include other gases such as hydrogen,nitrogen or inert gases. A preferred carbon containing gas is a mixtureof carbon monoxide and hydrogen. The carbon containing gas can furtherinclude an oxygen containing component. Preferred oxygen containingcomponents include CO₂, H₂O or O₂.

The carbon containing gas may be delivered to the reactor using anyconventional means. Furthermore, the carbon containing gas may bedelivered as a continuous flow to the reactor as the reaction is beingconducted continuously, or may be stored in the reactor prior to thereaction so that the reaction is conducted as a batch. The carboncontaining gas may further be preheated to a desired temperature beforedelivering to the reactor or before the reaction is conducted.

Supported Metal Catalyst

Catalytically active metals for producing single walled carbon nanotubesinclude metals from the Group VIII (e.g., Fe, Co, Ni, Ru, Rh, Pd, Os,Ir, Pt) or Group VIb (e.g., Cr, W, Mo) metals. Preferred metals includeFe, Co, Mn, Ni, W and Mo. Analogues or derivatives of the catalyticallyactive metals such as metal carbonyls (e.g., molybdenum carbonyls, ironcarbonyls, etc.) may be also used. Mixtures of any of the catalyticallyactive metals may also be used, including bimetallic or trimetalliccombinations.

To form a supported metal catalyst, the metal catalyst is deposited ontoa support using any conventionally known methods. Such methods typicallyinclude mixing, evaporating, and/or calcining the metal catalyst ormetal catalyst precursor in the presence of the support material. Othermethods include incipient wetness, impregnation, precipitation,co-precipitation, or chemical or physical adsorption. Alternatively, thesupport material may be contacted with or dipped into a solutioncontaining the metal catalyst or metal catalyst precursor, and thendried and/or calcined.

It is preferred to use aqueous solutions of Fe or Co salts essentiallyundissociated in water, especially Fe and Co carboxylates. Aqueoussolutions of Fe and Co acetate are particularly preferred. Mo is apreferred co-catalysts, also preferentially deposited from an aqueoussolution of Mo carboxylate.

The support may be made from alumina (Al₂O₃), or magnesia (MgO). Otheruseful support materials include silica (SiO₂), Mg(Al)O_(x), ZrO₂,molecular sieve zeolite, glass, quartz, clay, hydrotalcite, talc,aluminum foil, silicon and other known catalyst supports. It ispreferred that the surface of the support contain oxygen or oxidegroups. As such, a preferred support material is silica. The supportmaterial can be oxidized or otherwise treated using known methods todeposit oxygen containing groups or oxides onto the surface or planarsurface of the support material. For example, silicon can be oxidized attemperatures greater than 1000° C. to form or create a silica surface.

The support may be in the form of aggregates of tabular, prismatic orplatelet crystals. Alternatively, the support materials may consist ofspherical particles or aggregates lacking cleavable planar surfaces(e.g., Degussa fumed alumina). In the preferred embodiment, the supportis in the form of a platelet, wafer, or is of a form such that thesupport surface itself is planar (i.e., a planar substrate).

In the most preferred embodiment, the support material has at least oneoxidized planar surface.

Other types of support materials include carbon nanotube structures suchas carbon nanotube aggregates, three dimensional networks or rigidporous structures Carbon nanotube aggregates may be prepared by anyconventional methods, including those disclosed in U.S. Pat. No.5,165,909 to Tennent et al.; U.S. Pat. No. 5,456,897 to Moy et al.;Snyder et al., U.S. Pat. No. 5,707,916, filed May 1, 1991, and PCTApplication No. US89/00322, filed Jan. 28, 1989 (“Carbon Fibrils”) WO89/07163, and Moy et al., U.S. Pat. No. 5,456,897 filed Aug. 2, 1994 andPCT Application No. US90/05498, filed Sep. 27, 1990 (“Battery”) WO91/05089, and U.S. Pat. No. 5,500,200 to Mandeville et al., filed Jun.7, 1995 and U.S. Pat. No. 5,456,897 filed Aug. 2, 1994 and U.S. Pat. No.5,569,635 filed Oct. 11, 1994 by Moy et al, all of which are herebyincorporated by reference. Rigid porous structures may be made using anyconventional methods, including those disclosed in U.S. Pat. No.6,432,866 to Tennent et al., hereby incorporated by reference. Threedimensional networks may be made using any conventional methods,including those disclosed in U.S. Pat. No. 5,968,650 to Tennent et al.,hereby incorporated by reference.

Furthermore, before depositing or loading the metal catalyst, thesurface of the oxidized support material may be need to be pre-treatedto remove surface-adsorbed organics and/or moisture. One suchpretreatment method is to treat with an alcohol solvent such as ethanolor propanol. A preferred pretreatment method is to subject the supportmaterial to plasma treatment with gases such as oxygen. Other plasmasmay be used such as those based on F₂, O₂, NH₃, He, N₂ and H₂, otherchemically active or inert gases or mixture thereof. Such plasmatreatment may contribute to the oxidation of the surface.

Other known methods to increase the density of the oxygen groups on thesurface of the support material such as chemical treatment or additionalcalcination in air may be used.

Reaction Conditions

An important aspect in the process of the preferred embodiment is thatcommercially feasible yields of single walled carbon nanotubes can beproduced at reaction conditions (e.g., pressure, temperature) which arecommercially viable.

In the context of pressure, it has been discovered that the process ofthe preferred embodiment can be carried out at, about, or nearatmospheric pressure. This pressure condition would obviate the need fora vacuum or a pressure pump to artificially depressurize or pressurizethe reaction chamber. Vacuum operation is particularly disadvantageous:not only is there a danger of inleakage of atmospheric air leading to anexplosive situation, but the low density of sub-atmospheric gases limitsthe productivity per unit volume. Alternatively, the catalytic reactioncan be conducted at less than 10 atmosphere, between 0.5 to 10atmospheres, preferably between 1 to 5 atmospheres, or more preferablybetween 1 to 2 atmospheres.

Furthermore, in the context of temperature, it has been discovered thatthe process of the preferred embodiment can be carried out at relativelylower temperatures than those typical for forming carbon nanotubes viacatalytic decomposition reactions. Preferably, the reaction is carriedout at temperature below 800° C., more preferably between 500-750° C.,even more preferably between 550 to 650° C. Other possible temperatureranges include 500-700° C. or 550-700° C.

A continuous process is preferred. It should be understood that aprocess can be continuous on gas and still batchwise on catalyst andsolid phase products. A process continuous on gas phase adjusts the gasphase composition by separation steps external to the reaction zone andreturns the remaining gas to the reaction zone. The gas may be cooledprior to separating out the net gas phase products of reaction andbefore recompression. Obviously, compression energy is reduced if thefeed to the compressor is cooled. Before returning the recycle gas tothe reaction zone it may be reheated. Net gas feed to be consumed in thereactor may be added to the recycle gas or may be added separately tothe reactor.

Single wall carbon nanotubes can be efficiently produced by controllingthe oxidation potential in the reaction zone. A preferred method ofcontrolling the oxidation potential, where carbon monoxide is the carbonsource, is to control the amount of carbon dioxide in the reaction zone.Since CO₂ is a product of the desired reaction,2CO→C(SWT)+CO₂this can be accomplished by adjustment of the reaction zone feed rate,purge rate and recycle rate, all of which is well within the skill ofthe art. It is believed that the CO₂ reacts with undesirable amorphouscarbon which tends to poison the catalyst according to the reactionC (undesirable)+CO₂→2COand thereby returns CO to the reaction mixture.

Other sources of oxygen which may be used to reduce the amount ofundesirable carbon include molecular oxygen, N₂O and water.C(undesirable)+O₂→CO₂C(undesirable)+N₂O→N₂+COC(undesirable)+H₂O→H₂+CO

Use of water in a carbon monoxide based system, however, may alsoproduce hydrogen by the water gas shift reactionH₂O+CO→H₂+CO₂

In hydrocarbon based reactions, undesirable carbon forming on thecatalyst may be removed by maintaining a hydrogen partial pressure inexcess of the stoichiometric amount in the reaction being conductedhydrocarbon→C(SWT)+H₂These reactions are desirably carried out at non-vacuum, realisticoperating pressures as discussed above. Good hydrogenation catalysts,e.g. those containing Pd, Pt etc. may promote this effect. Additionally,hydrogen spillover, i.e. transfer of absorbed hydrogen from the metalcatalytic centers to the support may promote reaction with undesirablecarbon. Spillover is a function of both catalyst metal and support.

It must be understood that in a process continuous on gas phase, it ispossible to maintain a gas phase product of reaction at any desiredlevel in the reaction zone without “adding” said component. For example,if CO is the carbon source, any level of CO₂ can be maintained in thereaction zone by suitable adjustment if the downstream separation steps.Even if the desired oxidant is not a product of reaction, only thefraction of that additive oxidant lost in the recycle processing, needbe continuously added to the recycle or directly to the reactor.

The invention also includes methods and systems for harvesting singlewall tubes from catalysts comprising a non-porous support and asubstrate on which the single wall tubes have been grown. Generally, thesupported catalysts, including all substrates, have a thickness of lessthan about 0.5 mm and preferably less than about 0.1 mm. After thesingle wall nanotubes have grown on the catalyst, the tubes can beharvested by breaking up the product into smaller aggregates and furtherprocessing them as described below.

Harvesting can be performed in several ways. In one method, the tubesare separated from catalyst support within the reaction zone. In anothermethod they are separated from the catalyst support after the reactionstep has been completed. In both methods the solid-solid separation maybe performed using differential fluidization. In either method,recycling catalyst from which the single wall tubes have been separatedto the reaction zone may be advantageous.

In order to efficiently handle the supported catalyst and separate thetubes from it either within the reaction zone or in a subsequentseparation zone, it will be advantageous if the catalyst support is in acylindrical, spherical or cubic configuration. Desirably the cylindricalor spherical supported catalyst will have a minimum diameter of 0.25microns and a maximum diameter approximately equal to the length of thesingle wall tubes that are grown. Preferred supports may have a maximumdiameter of about 100 microns.

The lower level of supported catalyst particle diameter, 0.25 microns,is based upon the observation that non-porous catalyst particles havesufficient external surface area to serve as commercially usefulsubstrates for single walled carbon nanotube growth without a separateharvest step. The upper level of supported catalyst particle diameter isbased upon use of a separate harvest step and there the diameter is ofthe order of magnitude of the height of the SWTs grown on the externalsurface of the catalyst particle even though that limits the yield tosubstantially less than 100% allowing for density.

It is desirable to grow single walled carbon nanotubes of particularlength and aggregate size in order to simplify the steps of harvestingthe single walled carbon nanotubes and further processing them. Ingeneral, aggregates of more or less uniform size are easier to process.In addition, long, loosely-packed bundles of single walled carbonnanotubes can be avoided by limiting the length of the tubes.Accordingly, it is desirable to produce aggregates of single walledcarbon nanotubes of more or less uniform aggregate diameter having alength less than 1 cm and preferably less than 5 mm.

In one process embodiment the supported catalyst will remain in thereaction zone and the aggregates of single walled carbon nanotubes willabrade off the catalyst particles and be removed from the reaction zonein the product gas stream. The removal of the aggregates from thecatalyst particles by abrasion may be enhanced by including mechanicalelements within the reaction zone.

Where the process includes a separate harvest zone, the gas exiting thereaction zone may or may not be cooled before it enters the reactionzone. In either case catalyst and gas exiting from the harvest zone maybe recycled to the reaction zone. The catalyst may be first classifiedand a purge stream removed before it is recycled. Likewise a purgestream may be removed from the gas stream or it may be treated to removereaction products, e.g. CO₂ or H₂, before it is recycled.

Armed with the teachings of this application, other reaction conditions,such as reaction time, reactor size, etc., are all within the provinceof a skilled artisan to modify or adjust depending on the raw materialsand desired result. A reasonable number of experiments is envisioned tomaximize the yield with a particular carbon containing gas or supportedcatalyst, and are intended to fall within the scope of the preferredembodiment.

Raman Spectrum

Raman spectroscopy is a technique that enables one skilled in the art tocharacterize the materials under investigation. Conventionally, ingenerating a Raman spectrum, a particular wavelength of light, such as alaser beam, is shone onto the surface of the object. While most of thelight is reflected off unchanged, a small portion typically interactswith the molecules in the object and is scattered and produces the Ramaneffect, which is collected to produce a Raman spectrum. Differentmaterials have their own unique spectrum correlative to their presence,and thus, a Raman spectrum can be a useful analytical tool foridentifying materials.

As such, Raman spectra are commonly used to identify the forms of carbonpresent in a carbonaceous product based on the presence of certain peaksat certain regions in the spectra. For example, the region known as the“G-band” at ˜1580 cm⁻¹ is present in all types of graphite samples suchas highly oriented pyrolytic graphite (HOPG), pyrolytic graphite,charcoal as well as single walled and multi-walled carbon nanotubes. Aslight shift (˜15 cm⁻¹) towards higher wavenumber was observed forsamples with extremely small crystal sizes. The region known as the“D-band” (˜1355 cm⁻¹; however, the position of this band has been knownto depend strongly on the laser excitation wavelength) occurs when thematerial contains defects in the graphene planes or from the edges ofthe graphite crystal. The region known as “Radial breathing modes” or“RBM”, typically below 300 cm⁻¹ were observed in single walled carbonnanotubes, where all the carbon atoms under go an equal radialdisplacement. See Dresselhaus, M. S., et al., “Single Nanotube RamanSpectroscopy,” Accounts of Chemical Research I, vol. 35, no. 12, pp.1070-1078 (2002), hereby incorporated by reference.

In the preferred embodiment, the process yields a product which producesa Raman spectrum in which the ratio of the peak area of the G-band toD-band is at least higher than 2, with the presence of RBM.

Electron Microscopy

Another useful tool in analyzing the carbon product prepared from theprocess of the preferred embodiment is through electron microscopy. Inelectron microscopy, beams of electrons are irradiated onto the sample,and an image is produced based on the interaction between the electronsand the sample. In particular, two types of electron microscopes:transmission electron microscope (“TEM”) and scanning electronmicroscope (“SEM”) are commonly used to observe and characterize carbonnanotubes. Examples of the single walled carbon nanotubes produced inaccordance with the preferred embodiment are provided in FIGS. 3 and 4.

EXAMPLES

The following examples serve to provide further appreciation of theinvention but are not meant in any way to restrict the effective scopeof the invention.

Example 1 Preparation of Supported Catalyst

A silicon wafer was cut to 1 cm×2 cm, and put in an oven and calcined at1100° C. in air for 3-4 hours before being cooled to room temperature.After this treatment, the wafer exhibited dark blue color. The wafer wasthen cleaned in a ultrasonic bath containing 2-propanol for 5 minutesfollowed by air drying. The dried wafer was then treated in a minioxygen plasma reactor for 5 minutes. An ethanol solution composed of0.01 wt % Co and 0.01 wt % Mo was then deposited on this wafer via dipcoating. The coated wafer was then dried and calcined in air at 450° C.in air for one hour.

Example 2 Preparation of Supported Catalyst

10 grams of silica gel material (SiO₂) having a surface area of 400 m²/gis calcined in air at 400° C. for 3 hours and allowed to cool to roomtemperature in a round bottom flask. An ethanol solution containing Coacetate and Mo acetate with each metal content of 2.5 wt % is introducedto the SiO₂ via incipient wetness impregnation. The catalyst is thendried at 120° C. in air and followed by calcinations in air at 400° C.for 2 hours.

Example 3 Preparation of Supported Catalyst

10 grams of silica gel material having a surface area of 400 m²/g iscalcined in air at 400° C. for 3 hours and allowed to cool to roomtemperature in a glove-box and placed in a round bottom flask. Ananhydrous ethanol solution of ferrous ethoxide with Fe content of 5 wt %is introduced to the flask and allowed to react with the silica supportunder constant agitation for 5 hours. The slurry is then filtered, driedat 120° C. and calcined in air at 400° C. for 2 hours. The sample isthen further loaded with Mo by introducing an anhydrous ethanol solutionof Mo ethoxide containing 5 wt % Mo into a flask and allowed to reactwith the support under constant agitation for 5 hours. The slurry isthen filtered, dried at 120° C. and calcined in air at 400° C. for 2hours.

Example 4 Preparation of Single Walled Carbon Nanotubes

The catalyst made in Example 1 was placed in a 1-inch quartz reactor andpurged with argon for 30 minutes. 2% H₂/Ar replaced the purge gas whilethe reactor temperature was raised to 600° C. at 20° C./min. Once thetemperature reached 600° C., the H₂/Ar mixture was replaced by a CO flowat 400 mL/min, and the reaction was allowed to proceed for 30 minutes.After cooling to room temperature in 2% H₂/Ar, preliminary examinationof the wafer exhibited a black coating.

Example 5 Preparation of Single Walled Carbon Nanotubes

Procedure described in Example 4 is repeated for the catalyst made inExample 2.

Example 6 Preparation of Single Walled Carbon Nanotubes

Procedure described in Example 4 is repeated for the catalyst made inExample 3.

Example 7 Raman Spectrum

Raman spectrum of the product from Example 4 was recorded and arepresentative pattern was shown in FIG. 2, which exhibitedcharacteristic single-walled nanotube features.

Example 8 Electron Microscope

Sample 4 was subsequently examined by combination of SEM and HRTEM (Highresolution transmission electron microscope) to identify the morphologyof the products. Both studies indicated that the products are composedof single-walled carbon nanotubes with high purities and densities.These single-walled tubes are in the form of bundles or ropes that aresubstantially aligned and parallel to each other. The length of thesebundles is in the range of 1-2 μm, and diameters are in the range of 0.6to 1.5 nm.

Example 9 Supported Catalyst and Ethanol

Supported catalyst was prepared with a Fe loading of about 15 wt % andplaced into the reactor center at room temperature. 3% hydrogen in argonwas passed through the reactor while raising the reactor temperature to900° C. in about 30 minutes. Temperature of the reactor was lowered to700° C. to promote tube growth. Ethanol vapor at 0° C. was deliveredinto the tube. Raman spectra is shown as FIG. 5 and reveals no peak atthe RBM and large peak at the D-band.

Example 10 Supported Catalyst and Ethanol

Silicon wafers were oxidized with air at 1100° C. Wafers were sonicatedin propanol, cleaned with plasma and dip coated in 0.01% solution Coacetate and Mo acetate with ratio of 1:1 at 2 cm/min lifting speed. Thesupported catalysts were then calcined at 450° C. for 1 hour.

Three sets of experiments, A, B and C, were performed.

In experiment A, ethanol vapor was provided to the reactor at a pressureof 7 mm Hg. The reactant concentration was controlled by passing 1000mL/min 2% H₂/Ar through a liquid saturator containing ethanol kept at 0°C. and allowing just 70% of flow into the reactor via a gas splitter.The reaction was conducted at 800° C. for twenty minutes. Ramanspectrum, displayed in FIG. 6A, revealed moderate peak at RMB and smallpeak at D-band. SEM showed thick single walled carbon nanotube mat inthe order of 200-500 nanometers thick.

In experiment B, ethanol vapor was provided to the reactor at a pressureof 3 mm Hg. The reactant concentration was controlled by passing 1000mL/min 2% H₂/Ar through a liquid saturator containing ethanol kept at 0°C. and allowing just 30% of flow into the reactor via a gas splitter.The reaction was conducted at 700° C. Raman spectrum, also displayed inFIG. 6B, and SEM observation confirms strong signal with good singlewalled nanotube selectivity and clean product with little or noamorphous carbon.

In experiment C, ethanol vapor was provided to the reactor at a pressureof 1 mm Hg. The reactant concentration was controlled by passing 1000mL/min 2% H₂/Ar through a liquid saturator containing ethanol kept at 0°C. and allowing just 10% of flow into the reactor via a gas splitter.The reaction was conducted at 600° C. Raman spectrum, also displayed inFIG. 6C, showed weak signal at RBM.

Example 11 Supported Catalyst and Carbon Monoxide

Silicon wafers were oxidized with air at 1100° C. Wafers were sonicatedin propanol, cleaned with plasma and dip coated in 0.01% solution Coacetate and Mo acetate with ratio of 1:1 at 2 cm/min lifting speed. Thesupported catalysts were then calcined at 450° C. for 1 hour.

Two sets of experiments, A and B, were performed.

In experiment A, the supported catalysts were first reduced in thereactor at 700° C. with 2% H₂/Ar gas. CO gas was provided to reactor atrate of 400 ml/min for 30 minutes. Raman spectrum, displayed in FIG. 7A,revealed good peak at both RBM and G band, and small peak at D-band.

In experiment B, the supported catalysts were first reduced in thereactor at 600° C. with 2% H₂/Ar gas. CO gas was provided to reactor atrate of 400 ml/min for 30 minutes. Raman spectrum, displayed in FIG. 7B,revealed good peak at both RBM and G band, and small peak at D-band.

The Ramen spectra of from Experiments A and B were combined in FIG. 7Eand illustrates a greater growth of carbon products at 600° C. than 700°C.

Example 12 Supported Wafer and Carbon Containing Gas

A piece of silicon wafer is heated to over 1000° C. for several hoursand cooled to room temperature. The surface of the silicon wafer willturn blue. Other planar substrates may be used.

The wafer is dipped into an alcohol solution such as propanol andsonicated. The wafer is then treated with plasma.

The wafer is then dipped into a solution containing a metal catalyst.For example, a 0.01 wt % Co and 0.01 wt % Mo acetate solution. The waferis then calcined in an oven at temperatures in excess of 400° C. in air.

The calcined wafer catalyst is then placed into reactor and contactedwith a reduction gas mixture while the temperature is raised to adesired temperature range (e.g., 550-650° C.).

When the desired temperature is reached, the reduction gas mixture isreplaced with a carbon containing gas such as CO. The carbon containinggas may be preheated. The reactor is cooled. The same reduction gasmixture may be reintroduced into the reactor.

The resulting wafer is expected to be black. The Raman spectra isexpected to shows strong RBM and G band. SEM is expected to reveal cleansingle walled carbon nanotube growth.

The terms and expressions which have been employed are used as terms ofdescription and not of limitations, and there is no intention in the useof such terms or expressions of excluding any equivalents of thefeatures shown and described as portions thereof, it being recognizedthat various modifications are possible within the scope of theinvention.

1. A method for preparing single walled carbon nanotubes comprising the steps of: preparing a supported catalyst comprising a metal catalyst and a support material, said support material having at least one planar surface, calcining said supported catalyst at temperatures between 150 to 600° C., contacting said supported catalyst with a hydrocarbon and hydrogen at reaction conditions sufficient to produce at least 90% single walled carbon nanotubes in the amount greater than 1 gram single walled carbon nanotubes per gram metal catalyst, wherein the pressure in said reaction conditions is greater than about one and less than 10 atmospheres, the temperature is less than 800° C. and the partial pressure of hydrogen is maintained in excess of the stoichiometric level for the reaction taking place by selective control of the reaction feed, purge and recycle rates, and adjusting gas phase products of the reaction by separation steps to provide a recycle gas that is recycled to the contacting step.
 2. The method of claim 1, wherein the method is continuous. 